1. Field of the Invention
This invention relates generally to rolling mills producing hot rolled long products such as bars, rods, and the like, and is concerned in particular with a method of continuously rolling a product in consecutive upstream and downstream roll stands, with the product exiting from the upstream roll stand at a velocity that is higher than the take in velocity of the downstream roll stand.
2. Description of the Prior Art
In the typical rolling mill installation, billets are heated to an elevated rolling temperature in a furnace. The heated billets are then subjected to continuous rolling in successive roughing, intermediate and finishing sections of the mill, with each mill section being comprised of multiple roll stands. For larger products, the entire mill can usually be operated at or close to the maximum capacity of the furnace. However, when the rolling schedule calls for smaller products, the capacity of the finishing section is often reduced to well below that of the furnace and the roughing and intermediate mill sections. Under these circumstances, the roughing and intermediate sections can be slowed to match the capacity of the finishing section, but there are limits beyond which this becomes impractical. This is because acceptable rolling procedure dictates that the heated billets should be introduced into the first stand of the roughing section at a minimum take in speed, below which fire cracking of the rolls can take place.
In other cases, for example, when rolling high speed tool steels or nickel based alloys, a higher take in speed is required to avoid excess cooling of the billet, while lower finishing speeds are required to avoid excessive heat generation, which can cause core melting and surface cracking of the product.
These problems can be avoided by continuously rolling a product in consecutive upstream and downstream roll stands, e.g., the last stand of an intermediate mill section and the first stand of a mill finishing section, with the velocity of the product exiting from the upstream stand being higher than the take in velocity of the downstream stand, and with the excess product resulting from this velocity differential being temporarily accumulated between the two roll stands.
One prior attempt at achieving this objective is disclosed in U.S. Pat. No. 3,486,359 (Hein), where a laying head temporarily accumulates hot rolled products exiting from the intermediate mill section on a storage reel. The accumulated product is then unwound from the storage reel at a reduced speed for continued rolling in a mill finishing section. However, a number of drawbacks are associated with the Hein approach. For example, the product is not decelerated prior to being wound onto the storage reel. This, coupled with a lack of control over how the windings are distributed along the reel surface, can cause the windings to overlap one another, and this in turn can disrupt the unwinding process.
In U.S. Published application No. US2004-0250590A1 (Shore), a different system is disclosed for decelerating and temporarily accumulating a hot rolled product moving longitudinally along a receiving axis at a first velocity V1. The Shore system includes a continuously rotating laying assembly having an entry end aligned with the receiving axis to receive the product. The laying assembly has a curved intermediate section leading to delivery end that is spaced radially from the receiving axis and that is oriented to deliver the product in an exit direction transverse to the receiving axis. The curvature of the laying assembly and the orientation of its delivery end is such that the exiting product is formed into a helix. The helix is received and temporarily accumulated on a cylindrical drum arranged coaxially with the receiving axis. The drum is rotated continuously about the receiving axis in a direction opposite to the direction of rotation of the laying assembly and at a speed selected to unwind the accumulating helix at the velocity V3. The unwinding product is directed away from the drum by a catcher that is shiftable in a direction parallel to the receiving axis. During the time “T” required to roll a complete billet, a product length “L” equal to T×V2 is temporarily accumulated on the drum.
In the Shore system, the product is decelerated and formed into an ordered helix prior to being deposited on the drum. Product deceleration reduces the required storage capacity of the drum, and the ordered helix insures a smooth and trouble free unwinding of the product from the drum.
An essential requirement of the Shore system is accurate prediction of the time of arrival of the product front end at the delivery end of the continuously rotating laying assembly, coupled with precise synchronization of the rotating laying assembly with reference to the stationary catcher so as to insure smooth delivery of the product front end from the former to the latter.
The objective of the present invention is to provide an alternative method of operating the Shore system in which the laying assembly is stationary during delivery of product front ends to the catcher.